The present invention concerns emergency disconnection systems for loading systems, in particular for loading/unloading arms, for example for the transfer of liquefied natural gas.
More particularly, the invention concerns a system for emergency disconnection of a loading arm by means of two valves which are retained against each other using a collar.
Such disconnection systems are fully known from the state of the art, and there are several variants, described for example in the documents EP 0 748 429 B1 and U.S. Pat. No. 4,335,747.
In such a system, the disconnection procedure occurs in two steps: in a first phase, the valves are closed, and next the retaining collar is opened.
The closing of the valves and the opening of the collar are set off by one or more actuators. An actuator is a mechanical assembly adapted to operate the mechanisms for closing the valves and for opening the collar. These actuators must have a device for avoiding the opening of the collar before the valves have been fully closed, since this could have serious consequences (loss of part of the load, pollution, etc.). This device is termed an interlock in the technical field of the invention defined above.
A first type of emergency disconnection system comprises a first actuator for closing the valves and a second actuator (generally a hydraulic jack) for opening the collar. The opening of the collar is permitted by a hydraulic stop when the valves are closed. The security of the opening is thus managed by a hydraulic interlock. This type of interlock has several drawbacks: binding of the stop due to the corrosion induced by the sea air, the stop going out of adjustment during use.
If the hydraulic stop is not operational due to one of these problems, the opening of the collar cannot occur and the emergency disconnection system then becomes ineffective, which may prove dangerous.
The closing of the valves and the opening of the collar are moreover controlled therein by two dissociated solenoid valves which enables the valves to be closed without necessarily opening the collar, and to re-open the valves the case arising.
This system therefore makes it possible to perform a disconnection sequence in two steps that are independently controlled: closing of the valves, then opening of the collar.
Other types of disconnection systems comprise two jacks which are separate but integrated into the same actuator: a first jack for operating the valves and a second for opening the collar. These two jacks form an integral part of the actuator. This type of actuator has a mechanical interlock formed with two cams for example. It is thus impossible to operate the jack opening the collar while the valves are not completely closed. As in the preceding system, the closing of the valves and the opening of the collar are managed by two dissociated solenoid valves, which makes it possible to follow an emergency disconnection sequence in two independently controlled steps.
This system has drawbacks that are critical for emergency disconnection, in particular the high weight of the system, which, what is more, is situated at the end of the loading arm. Moreover, in certain embodiments, this system has the drawback of relatively slow actuation of the two separate jacks.
In addition, in certain embodiments, the second jack operates “empty” during the closing operation of the valves performed by the first jack.
A third type of system consists in closing the valves and opening the collar immediately after closing them. This type of system comprises a single jack actuator. The kinematics of the system are such that the jack closes the valves in a first phase and at the end of its travel, it enables the collar to be opened. Due to the kinematics of this actuator, it is impossible to open the collar without the valves being fully closed. The interlock is thus integrated into the actual kinematics of the system. However, this type of actuator does not enable the valves to be reopened once the disconnection procedure has been started, and in this case, the collar will necessarily be opened, since it is impossible to interrupt the travel of the jack until this has been completed, in particular on account of its inertia and construction. This type of system is described in particular in the document GB 2 162 270 A.
Here the disconnection sequence is controlled by a single solenoid valve.
However, in practice, a first alert threshold for the disconnection is set off. This first threshold corresponds to the closing of the valves. In case of higher risk, a second alert threshold is reached, and the opening of the collar is set off. If the second threshold is not reached, and the alert is cancelled, the valves are reopened and the loading procedure resumes.
Furthermore, this type of system is associated with a collar having two segments. A tie rod connecting those two collar segments is pivoted by the action of the jack at the end of travel, which makes it possible to free and to open those two segments. Thenceforth, the valves are no longer mechanically assembled and the emergency disconnection is achieved. However, this collar does not have a device for breaking the ice which would be formed around the collar during loading/unloading of cryogenic products, such as liquefied natural gas at approximately −165° C., as is the case on conventional collars with four radially opening segments. This ice which would be formed around the collar may prevent the two segments from completely opening despite the pivoting of the tie rod connecting the two segments under the action of the jack.